1. Field of the Invention
This invention relates to a method of artificially obtaining impact ionization coefficient rates of electron and hole in a semiconductor element using avalanche multiplication, such as a light-receiving element or a microwave oscillating element.
2. Description of the Prior Art
IMPATT diodes, avalanche photodiodes, etc. are known as elements using the avalanche multiplication by impact ionization. Also, it is known that the operation characteristics such as response speed, noise, sensitivity, etc. of these elements depend on the impact ionization coefficient rate .alpha./.beta. of electron and hole created by avalanche multiplication. Substances presently known as ones which cause such impact ionization phenomenon are very much limited semiconductor materials such as, for example, silicon and germanium, and the impact ionization coefficient rate thereof is a value inherent to the substance.
To optimize the operation of the element using said avalanche multiplication, it is necessary to set the impact ionization coefficient rate to a desired value, but the number of kinds of substances which cause the impact ionization is limited as described above. Accordingly, the impact ionization coefficient rates obtainable from the limited substances are also limited and therefore, there often occurs a case where a desired impact ionization coefficient rate cannot be obtained. For example, in avalanche photodiodes, it is known that the noise due to the amplification action during avalanche multiplication is minimum when the impact ionization coefficient rate .alpha./.beta. is infinite or zero. FIG. 1 of the accompanying drawings shows a graph in which the abscissa represents the inverse number of the square of the intensity of electric field and the ordinate represents the impact ionization coefficients .alpha..sub.si and .beta..sub.si of the electron and hole, respectively, of silicon. The impact ionization coefficient rate .alpha..sub.si /.beta..sub.si of silicon found from FIG. 1 is limited to the order of 4-6. The impact ionization coefficient rate is likewise limited to the order of 2-3 in the case of germanium, and if an avalanche photodiode is formed of these substances, there is a disadvantage of great noise.